This invention relates to a self-modulating scroll compressor that includes a built-in volume ratio that optimizes overall compressor performance.
Scroll compressors are becoming widely utilized in refrigerant compression applications. In a scroll compressor, a first scroll member has a base and a generally spiral wrap extending from the base. A second scroll member is held in a non-orbiting fashion relative to the first scroll member and has a wrap that interfits with a wrap from the first scroll member. The first scroll member is driven to orbit relative to the second, and the interfitting wraps define compression chambers for compressing an entrapped refrigerant.
It is a goal in modern compressor design to be able to provide at least two capacity levels. In some instances, such as when the cooling load on a refrigerant cycle is not particularly high, a lower capacity may be desirable. Less energy is used to compress a lesser amount of refrigerant in low capacity operations. Thus, various modulation schemes have been developed in the prior art.
In one modulation scheme, the compressor moves to low capacity operation when the pressure differential is low. The pressure differential is the delta (difference) of the discharge pressure to the suction pressure. When this quantity is low, there is some indication that lower capacity operation may be in order.
This prior art compressor performs adequately to provide low capacity operation when the compressor is utilized in an air conditioning cycle. However, it is also desirable to use such compressors as part of a heat pump system. In a compressor that is utilized for both air conditioning and heat pump operation, there are times when a relatively low pressure differential is not indicative of a need for low capacity. In particular, if the suction pressure is also low, the compressor may be operating in heat pump mode, and high capacity operation would still be desirable. As such, the prior art also provides for self-modulation between low capacity operation and high capacity operation to accommodate for those circumstances.
However, because the prior art scroll compressors are compression machines with a fixed volume ratio, the design of the compressor is only optimized for a single operating condition with a certain pressure ratio. For example, a typical scroll compressor for an air conditioning application has a built-in volume ratio of 1.9-2.6. The built-in volume ratio is defined as the ratio of a pockets' volume just after completion of suction, i.e. at the beginning of a compression cycle, to the pockets' volume just prior to discharge, i.e. at the end of the compression cycle.
Prior art self-modulating scroll compressors are typically capable of adjusting compressor capacity in two steps to achieve higher overall system efficiency. The prior art self-modulating scroll compressors reduce capacity by utilizing valves to bleed vapor from the pockets back to suction, which delays the completion of suction and significantly reduces the built-in volume ratio to an effective volume ratio. If, however, the effective volume ratio is too low at the lower capacity stage, the compressor efficiency will be reduced. As such, in order to have an optimized volume ratio at the lower capacity stage, the scroll built-in volume ratio at full capacity should be higher than that of the prior art single stage scrolls. Therefore, it is desirable to provide a self-modulated scroll compressor with a built-in volume ratio that is greater than found in the prior art.